Human cytomegalovirus (HCMV) is a betaherpesvirus which causes clinically serious disease in immunocompromised and immunosuppressed adults, as well as in some infants infected in in utero or perinatally (Alford and Britt, 1990). The 230-kb dsDNA genome of HCMV was sequenced (Chee et al., 1990) and has at least 200 open reading frames (ORFs). For purposes of this application, open reading frame is defined as the portion of a gene which encodes a string of amino acids and hence may encode a protein. The function of some HCMV proteins are known or predicted due to their homology with other viral (esp. herpes simplex virus) and cellular proteins. However, for the majority of the HCMV ORFs, the function(s) of the proteins they encode is unknown.
In order to study HCMV gene function HCMV deletion mutants can be constructed in order to assess their in vitro growth properties (Jones et al., 1991; Jones and Muzithras, 1992). For purposes of this application deletion mutants are defined as human cytomegalovirus mutants which lack regions of the wild-type viral genome. This strategy involves site-directed replacement mutagenesis of selected HCMV gene(s) by a prokaryotic reporter gene, usually .beta.-glucuronidase, although guanosine phosphoribosyltransferase can also be used. In this fashion, the recombinant virus can be isolated only if the replaced viral gene(s) is nonessential.
Several investigators have shown that infection by HCMV results in the down regulation of cellular MHC class I heavy chains (Browne et al., 1990; Beersma et al., 1993; Yamashita et al., 1993). For purposes of this application, down regulation is defined as reduction in either synthesis, stability or surface expression of MHC class I heavy chains. Such a phenomenon has been reported for some other DNA viruses, including adenovirus, murine cytomegalovirus, and herpes simplex virus (Anderson et al., 1985; Burget and Kvist, 1985; del Val et al., 1989; Campbell et al., 1992; Campbell and Slater, 1994; York et al., 1994). In the adenovirus and herpes simplex virus systems, the product of a viral gene which is dispensable for replication in vitro is sufficient to cause down regulation of MHC class I heavy chains (Anderson et al., 1985; Burget and Kvist, 1985). The gene(s) involved in class I heavy chain downregulation by murine cytomegalovirus have not yet been identified.